Normally when a slide projector is projecting the fan and bulb are
running simultaneously maintaining an even flow of air (or heat
exchange). If the projector is abruptly turned off, the theory is that
the heat in the bulb compartment will rise since there is no heat
exchange. It is this abnormal rise in temperature which will decrease
the life of the filament. Even when a bulb is first turned on it slowly
(a few seconds rises in temperature). The fan maintains a constant flow
of air giving the device a constant temperature. The heat exchange
process is just that. It maintains that given temperature without the
temperature rising abnormally as with one's radiator in theier car. As
you know some high reving cars will leave the radiator fan running for a
few minutes to maintain the temperature from rising after the engine is
turned off. It's not to cool the engine more quickly, its to prevent the
temperature from rising causing damage (in case of the cars) to their
aluminum heads. OK?
Phillip Franklin
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